I would say that one symbolic force in the novel would be the birds, themselves. The birds come to represent a force of malevolence. It is not merely that they are against the humans. The birds are set in seeking to destroy the humans. Nat has to fend off an entire flock that seems to have the obliteration of human beings at the forefront of their consciousness. In this light, the birds can come to symbolize the forces of negation that are present in human consciousness. How human beings react to these forces is of vital importance, as such a reaction defines what it means to be human. Nat does not relent in his need to survive and persevere. This is an example of the symbolic value of the birds. The birds operate as those forces that challenge human beings. They act as the forces of negation that humans beings must navigate and face throughout their consciousness. The birds operate as more than simply birds. Rather, they serve as a symbolic representation of the obstacles that test human resolve, presenting a challenging element of consciousness.